Friday, June 22, 2012

Bridges Bounce

Is bridges are flexible things.
The structure is made to move up and down,
Sometimes it feels they’re on springs.

Bill Emerson Bridge in Cape Girardeau, Mo.

My apologies to Tigger and his fans. I just couldn't help myself. ﻿

The idea for the verse popped in my head one evening as I waited for a crash to be cleared from my lane on a Missouri River bridge. As cars and commercial trucks rumbled by, I felt a bounce that was a bit unnerving. It wasn't the first time I'd felt the bounce. It happens on any type of bridge - overpasses and elevated lanes included.

Luckily, several bridge experts work just a short elevator ride from my desk. They helped me to understand that bridges are designed with some flexibility built in. They’ve got to perform in temperature extremes – contracting and expanding with cold and heat – in still and windy conditions, and under differing weight loads.

That flexibility means there will always be some movement. In fact – the Golden Gate Bridge in San Francisco is built to swing up to 27 feet mid-span in the unlikely event of a 100 mph broadside wind!

﻿

Setting the supports for the Miami Bridge.

It's hard to understand how something that looks so stable and solid could move up and down, but have you ever walked on a board laid across two supports? The further apart the supports are, the more vibration you feel as you walk across the board. This vibration, called resonance, also plays a part in bridge movement as vehicles move across a span.

Designers and engineers have to strike a careful balance of distances and supports, because a too-stiff bridge has its own issues.

﻿﻿If you are ever stopped on a span and become uneasy with the bouncing movement, maybe this will help - MoDOT's bridge inspection teams, who sometimes use a special truck to view the undersides of bridges, sheepishly tell stories of seasickness suffered when they spend too much time in the bucket attached to the flexible arm of the truck parked on a bouncing bridge. They might be uncomfortable, but they’re never scared. They know why bridges bounce. And now, so do you!
﻿

Inspectors examine the underside of the
Hurricane Deck Bridge using a snooper truck.

Missouri Department of Transportation

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